Tap water, groundwater, stream water, and the like are ingested as drinking water and also there is a possibility that they are unconsciously taken up from mouth, so that it is necessary to pay attention to their treatment in view of good hygiene. In addition to bacteria and suspended particulates, attention has been recently paid to protozoa such as Cryptosporidium. 
Cryptosporidium is a digestive tract-parasitic protozoan which is parasitic on mucous membranes of stomach and intestinal tracts to cause diarrhea. Cryptosporidium proliferates with repeating an asexual reproduction term and a sexual reproduction term and oocysts generated from the result of sexual reproduction are discharged into feces of parasitic hosts. Since the oocyst is stable and maintains activity for a long period of time, contamination of stream water and groundwater with oocysts discharged into for some reasons into drinking water may invite a situation that the oocyst infects human. In an infection example which occurred in Ogose-cho, Saitama-prefecture in 1996, a sewage-treatment plant exists at the upper stream of a river of raw water for tap water and thus oocyst as a primary infection source is considered to enter into tap water through a drinking-water treatment plant using the stream water as water source. Furthermore, discharged water from lavatories used by patients developing symptoms had been treated as sewage and again flew into the river and the stream water was utilized as a water source for tap water, so that infection was extended successively and finally a half of citizens were infected.
Oocyst of Cryptosporidium has an extremely strong resistance against disinfection by chlorine and treatment with ozone and thus it is impossible to completely annihilate the oocysts in water by usual water-purifying treatment. Therefore, in order to prevent infection with Cryptosporidium via water, it is necessary to assay a minute amount of oocysts in a sample in high accuracy together with sufficient removal or disinfection of the pathogenic protozoan.
Ministry of Health, Labor and Welfare has determined a guideline for tentative measure on preventive action and emergency action against these chlorine-resistant microorganisms (e.g., see Non-Patent Document 1). In the document, various methods for measuring Cryptosporidium are listed, including descriptions of an operating method comprising three steps: a “concentration step” where a collected sample is concentrated by one of the methods such as suction filtration, pressure filtration, cartridge filter method, or centrifugal precipitation, a “separation/purification step” where protozoan oocysts are separated from other suspending substances and purified by a method such as density-gradient centrifugal precipitation or immunological magnetic particle method (immunological magnetic bead method), and a subsequent “staining/microscopic inspection step” where the protozoan oocysts are immunologically stained and measured on a microscope; or an operating method comprising the above “concentration step” and “staining/microscopic inspection step”.
However, in the case that oocysts are separated from an environmental sample using a centrifugal means, specific gravity of Cryptosporidium is close to specific gravities of water and other contaminants and hence oocysts of Cryptosporidium present in the environmental sample cannot be completely recovered by conducting common low-speed centrifugal separation alone.
Moreover, in the guideline for tentative measure of Ministry of Health, Labor and Welfare, an immunological magnetic bead method is recommended as the above “separation/purification step” in the detection and measurement of oocysts of Cryptosporidium. This method is a method wherein immunological magnetic beads of 5 to 6 μm diameter are added to a sample subjected to the concentration step to effect an immune reaction and the oocysts bound onto the beads are magnetically recovered together with the beads. It may be convenient to observe the recovered oocysts directly through immunological fluorescent staining but actually, it is impossible to discriminate the oocysts because the immunological magnetic beads exhibit autofluorescence and also resemble the oocysts in size. Therefore, there is required a step of dissociating the oocysts from the magnetic beads having the oocysts bound thereto with hydrochloric acid. After the dissociation, a portion of the acid dissociation liquid containing the oocysts is placed on a glass slide and then neutralized with an alkali. After the solution of the preparation is air-dried, it is washed with methanol and then the oocysts are stained with fluorescent antibodies. The staining takes 30 minutes. Furthermore, in order to remove unbound fluorescent antibodies, washing is conducted but full attention should be paid so as not to wash out the oocysts from the preparation. Moreover, since a high-concentration salt is precipitated by the neutralization, the staining is not homogeneously effected and it is difficult to discriminate fluorescent oocysts from background in some cases. Also, in the operation for dissociation, all the oocysts are not always dissociated and some oocysts may remain on the magnetic beads, so that the recovery ratio is also problematic. Thus, the conventional method with the micron-size magnetic beads has defects that the steps are tedious and complex, a lot of skills are required for fluorescent staining, and also the recovery ratio of oocysts is insufficient.
Particularly, in the assay of tap water, since it is necessary to find one or two oocysts in a large amount of water, it is difficult even for those who attend skill-training to decide whether a substance emitting fluorescence belongs to Cryptosporidium or not and there are even confused cases induced by reports of mistaken detection, so that various problems remain on the method for detecting Cryptosporidium and thus water quality criteria therefor has not yet been defined. The reasons for requiring considerable skill in this assay method are that it takes a lot of time to concentrate Cryptosporidium from an analyte and that it requires the skill to discriminate Cryptosporidium oocysts under a fluorescent microscope.
As another staining/microscopic inspection step, anti-acid staining has been also developed but non-specific reaction with substances other than oocysts occurs remarkably in this method and thus there is a problem that the judgment of oocyst is difficult for a person who is not considerably skilled specialist.
As the other method for detecting Cryptosporidium, there has been known a method for detecting a specific DNA sequence of Cryptosporidium using a PCR process (e.g., see Patent Document 1). The method is a method wherein Cryptosporidium recovered by centrifugation is treated according to conventional procedures such as proteolytic treatment using proteinase K, phenol-chloroform treatment, and ethanol precipitation to recover DNA and then a sequence specific to Cryptosporidium is amplified by a PCR process to detect the presence of Cryptosporidium. 
However, this method employs a thermal cycler for amplifying DNA and a primer having a specific base sequence and hence is by no means an inexpensive and convenient method.
Furthermore, in both of the method using a specific antibody and a method of recognizing a specific DNA sequence by a PCR process, centrifugation is used at the time when Cryptosporidium is recovered. However, since specific gravities of oocysts and sporozoites of Cryptosporidium are near to 1, there is a large loss part impossible to recover by general low-speed centrifugation and hence there is a problem that Cryptosporidium present in a sample cannot be sufficiently detected.
Non-Patent Document 1: Measure against Cryptosporidium in Tap Water, supervised by Ministry of Health and Welfare, Life Hygiene Bureau, Water Environment Division, Water Maintenance Department, published by K. K. Gyousei (December, 1999), pp. 1-2.
Patent Document 1: JP-A-11-243953